Ink-jet printer and ink jetting method

ABSTRACT

In an ink-jet head including black, yellow, magenta, cyan nozzle rows, the nozzle rows are arranged so that in each of the nozzle rows, adjacent nozzles are arranged by a predetermined spacing distance P, and that the black nozzle row is shifted in a nozzle-row alignment direction by a spacing distance smaller than the predetermined spacing distance P, with respect to the cyan, magenta and yellow nozzle rows. When a black dot is formed between monochrome-black dots adjacent in the sub-scanning direction, a dot of color-mixed black which is a mixture of the other three colors is formed between the monochrome-black dots. At this time, by adjusting a black-ink jetting amount to be greater than a total amount of the other three color inks, it is possible to make the monochrome-black dot and the color-mixed black dot to be mutually uniform in size.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent ApplicationNo. 2005-360407, filed on Dec. 14, 2005, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink-jet printer which jets(discharges) ink onto a recording medium, particularly to an ink-jetprinter capable of jetting yellow, magenta, and cyan inks, which areinks of three primary colors, and a black ink, and the present inventionrelates to an ink jetting method.

2. Description of the Related Art

Conventionally, as ink-jet printers, those of a type capable ofperforming color recording have been in wide use, and there has beenknown an ink-jet printer which jets not only a black ink but alsoyellow, magenta, and cyan inks which are inks of three primary colorsindependently onto a recording medium. It has been also known to utilizepseudo black color (sepia color; hereinafter referred to as color-mixedblack) which is a mixture of a yellow ink, a magenta ink, and a cyan inkjetted onto a recording medium at an identical or same position thereof.

In Japanese Patent Publication No. 3533771, there is described astructure in which nozzle rows of (for) a black ink, a yellow ink, amagenta ink, and a cyan ink respectively are arranged in parallel to oneanother along a direction (sub-scanning direction) orthogonal to ascanning direction of an ink-jet head, with a predeterminednozzle-arranging spacing distance at which nozzles are arranged in eachof the nozzle rows, and in which only the nozzle row for (associatedwith) the black ink is shifted (deviated), by ½ of the nozzle-arrangingspacing distance, from the other nozzle rows in the sub-scanningdirection. To form dots continuously in the sub-scanning direction byusing only dots of the black ink (black-ink dots), the ink-jet headneeds to run (perform scanning) twice in the scanning direction, with arecording medium shifted by ½ of the nozzle-arranging spacing distance.On the other hand, in the structure described in the above-describedJapanese Patent Publication No. 3533771, by making the ink-jet head runonly once in the scanning direction, a gap (space) between dots of theblack ink (monochrome black) (monochrome-black-dots) which are adjacentin the sub-scanning direction can be filled with a dot of thecolor-mixed black (color-mixed black dot) which is a mixture of theyellow ink, the magenta ink, and the cyan ink. Therefore, it is possibleto improve pixel density and increase recording speed in monochromaticrecording.

However, the inventor has found out that in a case in which dots of thecolor-mixed black (color-mixed black dots) are thus recorded amongmonochrome-black dots, the dots are nonuniformly formed. Suchnonuniformity of the dots adversely affects printing quality.

SUMMARY OF THE INVENTION

The present invention solves the above problems, and an object of thepresent invention is to improve uniformity of dots formed on a recordingmedium so as to obtain satisfactory recording quality, in recordingwhich uses color-mixed black obtained by jetting and mixing a yellowink, a magenta ink, and a cyan ink. It should be noted thatparenthesized reference numerals and symbols assigned to elements shownbelow are merely examples of the elements and are not intended to limitthe elements.

According to a first aspect of the present invention, there is providedan ink-jet printer (1) which performs recording on a recording medium(PA) by jetting a black ink (Bk), a yellow ink (Y), a magenta ink (M),and a cyan ink (C), the ink-jet printer (1) including:

an ink-jet head (3) including black, cyan, magenta and yellow nozzlerows (15 a to 15 d) each of which is formed of a plurality of nozzles(15) arranged in a predetermined direction, and which are aligned in adirection orthogonal to the predetermined direction; and

a controller (30) which controls the ink-jet head (3);

wherein adjacent nozzles (15), among the nozzles (15) in each of thenozzle rows (15 a to 15 d) are arranged at a predetermined spacingdistance P;

the black nozzle row (15 a) is shifted, in the predetermined directionby a spacing distance ΔP smaller than the predetermined spacing distanceP, from the cyan, magenta and yellow nozzle rows (15 b to 15 d),respectively;

when a black dot is formed between monochrome-black dots (A1) ofmonochrome black which are adjacent to each other in the predetermineddirection and which are formed by jetting the black ink (Bk), thecontroller (30) controls the ink-jet head (3) so that the yellow ink(Y), the magenta ink (M), and the cyan ink (C) are jetted from nozzles(15) to form a color-mixed black dot (B1) of color-mixed black which isa mixture of the yellow ink (Y), the magenta ink (M), and the cyan ink(C); and

the controller (30) controls the ink-jet head (3) so that an amount ofthe black ink (Bk) which is jetted to form one dot of themonochrome-black dots (A1) is greater than a total amount of the yellowink (Y), the magenta ink (M), and the cyan ink (C) jetted to form thecolor-mixed black dot (B1).

In a case in which the color-mixed black dot or dots (B1) are recordedamong the monochrome-black dots (A1) in a mixed manner, if the amountsof the inks jetted from the nozzles (15) jetting the four color inksrespectively are set equal to one another, an ink amount (volume) forthe color-mixed black dot (B1) which is a mixture of the three colorinks (Y, M, C) jetted from the three nozzles (15) respectively becomesgreater than an ink amount for (per) the monochrome black-dot (A1).Further, in a case in which a pigment ink is used as the black ink (Bk)and dye inks are used as the three color inks (Y, M, C) other than theblack ink (Bk), in order to increase sharpness of black in characterrecording and the like and to make color vivid, the monochrome-black dot(A1) and the color-mixed black dots (B1) become nonuniform in sizebecause a dot of the pigment ink, which hardly blur, is formed as a dotwhich is smaller on the recording medium (PA) than a dot of the dye inkeven if the pigment ink and the dye ink are jetted in a same amount(volume). With respect to this situation, the inventor has found outthat it is possible to improve the recording quality by making the dotsformed on the recording medium (PA) uniform in size. According to thepresent invention, the amount of the black ink (Bk) jetted to form onemonochrome-black dot (A1) is made greater than the total amount of theyellow ink (Y), the magenta ink (M), and the cyan ink (C) jetted to formone color-mixed black dot (B1). Accordingly, in a case in which thecolor-mixed black dots (B1) and the monochrome-black dots (A1) bothexist in a mixed manner, it is possible to make the dots uniform in sizeon the recording medium (PA). Further, whereas the adjacent nozzles (15)in each of the nozzle rows (15 b to 15 d), which are different from thenozzle row (15 a) for the black ink, are arranged in the predetermineddirection at the predetermined spacing distance P, only the black nozzlerow (15 a) for the black ink (Bk) is shifted from the other nozzle rows(15 b to 15 d) in the predetermined direction by the spacing distance ΔPwhich is smaller than the predetermined spacing distance P. Therefore,when the ink-jet head (3) is made to run once with respect to therecording medium (PA) in the direction orthogonal to the predetermineddirection, it is possible to form the color-mixed black dot (B1) byjetting the yellow ink (Y), the magenta ink (M), and the cyan ink (C)between the monochrome-black dots (A1), which are adjacent in thepredetermined direction, as necessary, thereby making it possible toimprove the pixel density.

In the ink-jet printer (1) of the present invention, the black nozzlerow (15 a) may be shifted, in the predetermined direction by ½ of thepredetermined spacing distance P, from the cyan, magenta and yellownozzle rows (15 b to 15 d), respectively. In this case, by making acarriage (9) run only once in the scanning direction, it is possible toform a color-mixed black dot (B1) at a position between monochrome-blackdots (A1) adjacent in the predetermined direction so that themonochrome-black dots (A1) and the color-mixed black dot (B1) are madeto be continuous in the predetermined direction, which consequently canimprove the recording speed.

In the ink-jet printer (1) of the present invention, the nozzles (15)which form the black nozzle row (15 a) and which jets the black ink (Bk)may have an opening which is greater in size than that of the nozzles(15) which form the cyan, magenta and yellow nozzle rows (15 b to 15 d)and which jet the cyan ink (C), the magenta ink (M), and the yellow ink(Y) respectively. In this case, an amount of the black ink (Bk) jettedfrom each of the nozzles (15) for the black ink (Bk) can be made greaterthan an amount in which each of the yellow ink (Y), the magenta ink (M),and the cyan ink (C) is jetted from the nozzles (15) for these inks,respectively. Therefore, in a case in which the color-mixed black dots(B1) and the monochrome-black dots (A1) both exit in a mixed manner, itis possible to make the dots uniform in size on the recording medium(PA).

In the ink-jet printer (1) of the present invention, when thecolor-mixed black dot (B1) is formed between the monochrome-black dots(A1), an amount of the yellow ink (Y), an amount of the magenta ink (M),and an amount of the cyan ink (C) jetted to form the color-mixed blackdot (B1) may each be adjusted to be smaller than the amount of the blackink (Bk) jetted to form one dot of the monochrome-black dots (A1). Xnthis case, the color-mixed black dot (B1) and the monochrome-black dot(A1) which are formed on the recording medium (PA) can be formed in asubstantially equal area size or dimension, thereby making it possibleto improve the recording quality. Furthermore, since each of the amountof the yellow ink (Y), the amount of the magenta ink (M), and the amountof the cyan ink (C) jetted to form the color-mixed black dot (B1) isreduced, electrical driving power applied to a piezoelectric actuator(17) can be reduced and thus the heat generation of the piezoelectricactuator (17) can also be reduced.

In the ink-jet printer (1) of the present invention, an ink, which blursmore easily with respect to the recording medium (PA) than the black ink(Bk), may be selected in advance as each of the yellow ink (Y), themagenta ink (M), and the cyan ink (C). As a result, the color-mixedblack dot (B1) which easily blurs and the monochrome-black dot (A1)which blurs in an extent smaller than the color-mixed black dot (B1) canbe made uniform in size, thereby making it possible to realizesatisfactory recording quality.

In the ink-jet printer (1) of the present invention, the amount of theblack ink (Bk) jetted to form one dot of the monochrome-black dots (A1)may be 1.3 to 1.5 times an amount of each of the yellow, magenta, andcyan inks (Y, M, C) jetted to form one dot of each of the yellow,magenta, and cyan inks in a size same as that of one dot of themonochrome-black dots (A1). Generally, a pigment ink is used as theblack ink (Bk) and dye inks are used as the color inks (Y, M, C) in manycases, and since the pigment ink blurs to a smaller extent as comparedto the dye ink. Accordingly, by jetting the black ink (Bk) in an amountgreater than an amount of each of the color inks (Y, M, C), it ispossible to make the black dot (A1) and the dot of each of the colorsuniform in size.

In the ink-jet printer (1) of the present invention, when thecolor-mixed black dot (B1) is formed between the monochrome-black dots(A1), the amount of the black ink (Bk) jetted to form one dot of themonochrome-black dots (A1) may be smaller than a maximum ink amountjettable for forming one dot of the monochrome-black dots (A1) from eachof the nozzles (15) which form the black nozzle row (15 a) and which jetthe black ink (Bk). By using the color-mixed black dot (B1), it ispossible to form, on the recording medium (PA), a black-color region(black-color area) which is continuous at least in the sub-scanningdirection without jetting the ink droplets each in the maximum inkamount jettable from each of the nozzles (15) for the black ink (Bk). Tojet the ink droplet in the maximum ink amount, electrical driving powerapplied to the piezoelectric actuator (17) also becomes high. However,by adjusting the ink amount, in which the black ink is to be jetted, toan amount smaller than the maximum ink volume, it is possible to reducethe electrical power, and to reduce heat generation of the piezoelectricactuator (17) as well.

According to a second aspect of the present invention, there is providedan ink jetting method for jetting a black ink (Bk), a yellow ink (Y), amagenta ink (M), and a cyan ink (C), the method including:

preparing an ink-jet head (3) which includes black, cyan, magenta andyellow nozzle rows (15 a to 15 d) each formed of a plurality of nozzles(15) arranged in a predetermined direction at a predetermined spacingdistance P, and are aligned in a direction orthogonal to thepredetermined direction, and in which the black nozzle row (15 a) isshifted in the predetermined direction by a spacing distance ΔP smallerthan the predetermined spacing distance P, from the cyan, magenta andyellow nozzle rows (15 b to 15 d), respectively;

jetting the yellow ink, the magenta ink, the cyan ink concurrently so asto form a color-mixed black dot (B1) of color-mixed black which is amixture of the yellow ink (Y), the magenta ink (M), and the cyan ink(C), when a black dot is formed between monochrome-black dots (A1) ofmonochrome black which are adjacent to each other in the predetermineddirection and which are formed by jetting the black ink (Bk); and

adjusting an amount of the black ink (Bk) to be greater than a totalamount of the yellow ink (Y), the magenta ink (M), and the cyan ink (C)jetted for forming the color-mixed black dot (B1), when one dot of themonochrome-black dots (A1) is formed.

According to this method, the amount of the black ink (Bk) to be jettedis made greater than the total amount of the color inks (yellow, magentaand cyan inks) for forming the color-mixed black dot (B1). Accordingly,in a case in which the color-mixed black dots (B1) and themonochrome-black dots (A1) both exit in a mixed manner, it is possibleto make the dots on the recording medium (PA) uniform in size. Further,whereas the adjacent nozzles (15), in each of the nozzle rows (15 b to15 d) for one of the color inks other than the black ink, are arrangedin the sub-scanning direction at the predetermined spacing distance P,only the black nozzle row (15 a) for the black ink (Bk) is shifted, fromthe nozzle rows (15 b to 15 d), in the sub-scanning direction by thespacing distance Δp which is smaller than the predetermined spacingdistance P. Therefore, when the ink-jet head (3) is made to run once inthe scanning direction with respect to the recording medium (PA), it ispossible to form a color-mixed black dot (B1) by jetting the yellow ink(Y), the magenta ink (M), and the cyan ink (C) between themonochrome-black dots (A1) adjacent in the sub-scanning direction, asnecessary, thereby making it possible to improve the pixel density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of main parts or components of an ink-jet printer;

FIG. 2 is an exploded perspective view of an ink-jet head;

FIG. 3 is a view showing a nozzle arrangement in a first embodiment;

FIG. 4 is a view explaining a recording state in the first embodiment ina partial manner;

FIG. 5 is table showing an example of ink jetting amounts for formingdots mutually different in size;

FIG. 6A and FIG. 6B are views explaining another recording state in thefirst embodiment in a partial manner;

FIG. 7 is a view showing a nozzle arrangement in a second embodiment;

FIG. 8A and FIG. 8B are views explaining a recording state in the secondembodiment in a partial manner;

FIG. 9 is a block diagram showing a major structure of a control systemof the ink-jet printer; and

FIG. 10 is a flowchart showing a flow of an ink jetting process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, basic embodiments of the present invention will beexplained based on the drawings.

An ink-jet printer 1, as shown in FIG. 1, of this embodiment (firstembodiment) is applicable, for example, not only to a printer devicehaving only a printer function but also to a Multi Function Device (MFD)including, in addition to the printer function, a copy function, ascanner functions a facsimile function, and so on. The ink-jet printer 1has an ink-jet head 3 and a carriage 9. The ink-jet head 3 is providedin a body frame 2 of the ink-jet printer 1 and mounted on the carriage 9so as to perform recording by jetting an ink to a paper PA as arecording medium, and the ink-jet head 3 is constructed to be movablealong a main scanning direction (Y direction).

The carriage 9 is slidably arranged on a rear guide shaft 6 and a frontguide shaft 7 which are provided on the body frame 2 in parallel to eachother along the Y direction. The carriage 9 is reciprocatable in themain scanning direction (Y direction) along the front and rear guideshafts 6, 7 by a carriage motor 10 which is arranged in the body frame 2on the right rear side thereof and by a timing belt 11 which is anendless belt. The paper PA is transported or fed, by a known papertransporting mechanism (not shown), in a horizontal manner across aspace below or under the lower side of the ink-jet head 3, along asub-scanning direction (X direction) which is orthogonal to the mainscanning direction (direction indicated by an arrow A in FIG. 1. Toperform a recording, the ink is jetted onto the paper PA from nozzles 15(see FIG. 3) of the ink-jet head 3 which is attached to the carriage 9and moves in the main scanning direction (Y direction).

As shown in FIG. 9, the ink-jet printer 1 is provided with a controller30 which controls the scanning performed by the ink-jet head 3, thefeeding of the paper PA in the sub-scanning direction, and the jettingof the ink from each of the nozzles 15. An operation panel 31 throughwhich a user gives an instruction for a recording mode and the like; amotor driver 32 for driving the carriage motor 10; a motor driver 34 fordriving an LF motor 33; a paper sensor 35 which detects a leading end ofthe paper FA; and an origin sensor 36 which detects an origin positionof the carriage 9 are connected to the controller 30. The controller 30controls the ink-jet head 3 to adjust a jetting amount of the ink jettedfrom each of the nozzles 15. For this purpose, a plurality of kinds ofdriving waveforms of driving voltages, applied to a piezoelectricactuator 17, are prepared in the controller 30 according to ink jettingamounts.

The ink-jet head 3 is driven by a driving IC 40, and the driving IC 40is controlled by a gate array (G/A) 41. Electrodes constructingpiezoelectric elements, respectively, provided in the ink-jet head 3 areconnected to the driving IC 40, and based on the control by the gatearray 41, the driving IC 40 generates a driving signal suitable for theink-jet head 3 to apply the generated driving signal to each of theelectrodes.

The controller 30 and the gate array 41 are connected to each other viaan address bus 60 and a data bus 61. The controller 30 generates arecording timing signal TS and a control signal RS according to aprogram pre-stored in the controller 30, and transfers or transmits thesignals TS, RS to the gate array 41. In accordance with the recordingtiming signal TS and the control signal RS and based on recording datastored in an image memory 42, the gate array 41 generates transfer datasignal DATA for recording the recording data onto the paper PA, atransfer clock TCK which is synchronized with the transfer data signalDATA, a strobe signal STB, and a recording clock ICK, and the gate array41 transmits these signals DATA, TCK, STB, ICK to the driving IC 40.

Further, the gate array 41 makes the image memory 42 store thereinrecording data transmitted from an external apparatus such as a hostcomputer (host PC) 43. Based on the data transmitted from the hostcomputer 43 or the like, the gate array 41 generates a datareception/interruption signal WS and transmits the generated signal WSto the controller 30. Further, an encoder sensor 44, which detects arunning position of the carriage 9, is connected to the gate array 41.

Note that an ink jetting operation from the nozzles 15 is performed, asgenerally known, such that while the carriage 9 is moving in the mainscanning direction (Y direction) at a constant speed in a state wherethe paper PA is stopped; after the scanning is performed once (onetime), the paper PA is fed or transported in the sub-scanning directionby a predetermined feed amount; and then the next scanning is performedin a similar manner.

In the body frame 2, an ink tank 5 a storing a black ink (Bk), an inktank 5 b storing a cyan ink (C), an ink tank 5 c storing a magenta ink(M), and an ink tank 5 d storing a yellow ink (Y) are stationary placed,and the inks in the ink tanks are supplied to the ink-jet head 3 viaflexible ink supply tubes 14 a to 14 d respectively.

The ink-jet head 3 is attached to the carriage 9 so that the nozzles 15(see FIG. 3) are open and exposed on the side of the lower surface ofthe carriage 9. As shown in FIG. 2, the ink-jet head 3 is constructed ofa cavity unit 16 which includes the plurality of nozzles 15 and inkchannels 19 corresponding to the nozzles 15 respectively; thepiezoelectric actuator 17 which applies a jetting pressure to the inksin the ink channels 19; and a flexible flat cable 18 for applying anelectric signal to the piezoelectric actuator 17.

As shown in FIG. 3, the nozzles 15, each of which jets one color inkamong the black, cyan, magenta, and yellow inks, are arranged in rows inthe sub-scanning direction (predetermined direction: X direction) toform nozzle rows, and the nozzle rows are aligned in the main scanningdirection (direction orthogonal to the predetermined direction: Ydirection) corresponding to the colors of the inks, respectively, suchthat at least one nozzle row, among the nozzle rows, for one of theblack, cyan, magenta and yellow inks. In this embodiment, a nozzle row15 a of the black ink (black nozzle row 15 a), a nozzle row 15 b of thecyan ink (cyan nozzle row 15 b), a nozzle row 15 c of the magenta ink(magenta nozzle row 15 c), and a nozzle row 15 d of the yellow ink(yellow nozzle row 15 d) are arranged in the main scanning direction (Ydirection) such that these four nozzle rows correspond to the fourcolors respectively. In each of the nozzle rows corresponding to one ofthe colors, the nozzles 15 are arranged in a zigzag or staggered form;and in each of the nozzle rows, nozzles 15 among the nozzle rows 15which are adjacent in the sub-scanning direction (X direction) arearranged at a predetermined spacing distance P In each of the nozzlerows, nozzles 15, among the nozzles 15, positioned at both ends thereofrespectively is arranged at a spacing distance “L”. Further, thepredetermined spacing distance P corresponds to a recording resolutionNdpi (dot per inch, N is a positive integer) in the sub-scanningdirection (X direction).

In this embodiment, the black nozzle row 15 a is shifted, from the cyannozzle row 15 b, the magenta nozzle row 15 c, and the yellow nozzle row15 d, in the sub-scanning direction (X direction) by an amountcorresponding to ½ (half) of the predetermined spacing distance P. Ineach of the cyan nozzle row 15 b, the magenta nozzle row 15 c, and theyellow nozzle row 15 d are aligned in the main scanning direction (Ydirection), the nozzles 15 are arranged along a straight line.Therefore, in a case in which the resolution in the sub-scanningdirection (X direction) of the nozzles 15 in each of the nozzle rows isNdpi (N is a positive integer), then the nozzles 15 of the black ink andthe nozzles 15 of the color inks other than the black ink are arrangedso that the resolution in the sub-scanning direction therebetween is 2Ndpi. Here, as an example, it is assumed that the resolution in thesub-scanning direction is 150 dpi in each of the nozzle rowscorresponding to the colors respectively, then the resolution in thesub-scanning direction between the nozzles 15 of the black ink and thenozzles 15 of the other color inks is 300 dpi.

With respect to the nozzles 15 of the respective colors, an opening size(orifice size) of the nozzles 15 of the black ink is set to be greaterin advance than that of the nozzles 15 of the other three colors, andthe opening size of the nozzles 15 of the black ink may be 20.5±1 μm,and the opening size of the nozzles 15 of the other three colors may be17.0±1 μm. Note that in this embodiment, whereas the opening size of thenozzles 15 of the black ink is set to be 20.5 μm, the opening size ofthe nozzles 15 of the other three colors is set to be 17.0 μm. Theopening size of the nozzles 15 of the other three color inks (X, M, C)is set to be smaller than the opening size of the nozzles 15 of theblack ink. Consequently, when the nozzles 15 for the black ink and thenozzles 15 for the other three colors are driven by the piezoelectricactuator 17 in a same manner, each of the nozzles 15 of the other threecolors can jet an ink droplet in an amount smaller than an amount inwhich an ink droplet jetted from the nozzles 15 of the black ink.

An ink jetting amount per dot jetted from each of the nozzles 15 isadjustable by controlling a driving manner of the piezoelectric actuator17 as publicly known. Therefore, a plurality of kinds of drivingwaveforms are prepared in advance in the controller 30 in order to jet aplurality of ink droplets which are mutually different in ink jettingamount, and based on data to be recorded, the controller 30 selects anoptimum driving waveform, among the driving waveforms, to be applied toan active portion, of the piezoelectric actuator 17, corresponding toeach of the nozzles 15. FIG. 5 shows, as an example, ink jetting amountsto form dots which are different in size, and three kinds of ink dotsizes referred to as large, intermediate, and small are prepared for thenozzles 15 of the black ink and for the nozzles 15 of the other threecolor inks (Y, M, C). To form a large dot of black (large-black dot),the black ink is jetted in an amount of 24 pl, whereas an amount inwhich the yellow ink is jetted to form a large dot of yellow(large-yellow dot), an amount in which a magenta ink is jetted to form alarge dot of magenta (large-magenta dot), and an amount in which thecyan ink is jetted to form a large dot of cyan (large-cyan dot) areadjusted to be 16 pl, respectively. Further, to form the intermediatedot, the black ink is jetted in an amount of 7 pl, whereas an amount inwhich the yellow ink is jetted, an amount in which the magenta ink isjetted, and an amount in which the cyan ink is jetted are 5 pl,respectively. To form the small dot, an amount in which the black ink isjetted is 4 pl, whereas an amount in which the yellow ink is jetted, anamount in which the magenta ink is jetted, and an amount in which thecyan ink is jetted are 3 pl, respectively. Generally, a pigment ink isused as the black ink and dye inks are used as the color inks in manycases. Since the pigment ink blurs to an extent smaller as compared tothe dye ink, an amount in which the black ink is jetted to form one dot(one black dot) is adjusted 1.3 to 1.5 times the amount in which each ofthe color inks is jetted to form one dot of the yellow, magenta and cyanwith the same size as the one dot of the black ink. Although not shownin FIG. 5, it is also possible to form an extra-large dot by jetting theink in an amount greater than the amount for forming a large dot.

Next, a recording operation performed by the ink-jet printer 1 asstructured above will be explained by using FIG. 4 showing a recordingstate in a partially manner. In FIG. 4, dots of monochrome black(monochrome-black dots) by the black ink are indicated as “A1”; and dotsof color-mixed black (color-mixed black dots) which is a mixture of theyellow ink, the magenta ink, and the cyan ink jetted to the sameposition of the recording medium PA are indicated as “B1”. In theink-jet printer 1, when black dots are to be further formed between themonochrome-black dots A1 which are adjacent to each other in thesub-scanning direction (X direction) and which are formed by jetting theblack ink, then the controller 30 drives the ink-jet head 3 via thedriving IC 40 so that the yellow ink, the magenta ink, and the cyan inkare jetted, thereby forming the color-mixed black dots B1 each of whichis a mixture of the yellow ink, the magenta ink, and the cyan ink.

Namely, in the ink-jet printer 1, in a case in which data to be recordedincludes a solid black area which is continuous at least in thesub-scanning direction or in a case in which black dots need to beformed with 2 Ndpi (300 dpi) resolution in the sub-scanning direction,then the timing at which the inks are jetted from the nozzles 15(jetting timing of the nozzles 15) is controlled so that the color-mixedblack dots B1 are formed, between the monochrome-black dots A1 which areadjacent in the sub-scanning direction (X direction), by making theink-jet head perform one running (one pass) in the main scanningdirection (Y direction). Note that although FIG. 4 shows each one of thedots with a small area dimension in order to clearly show thearrangement of the dots, but in an actual printing, each of the dots hasan area dimension to an extent such that adjacent dots overlap with eachother without any space (gap) existing therebetween.

Consequently, only by making the ink-jet head 3 perform only one runningin the main scanning direction (Y direction), it is possible to form,with 2 Ndpi (300 dpi) resolution, a black area continuous in thesub-scanning direction. Of course, in order to form the black areacontinuous also in the main scanning direction (Y direction), it isallowable that the inks are jetted (jetted at continuous driving cycles)so that monochrome-black dots A1 and color-mixed black dots B1 are bothcontinuously formed in the main scanning direction (Y direction).Further, in order to form the black area still longer in thesub-scanning direction, then it is allowable to perform an operation inwhich, after the paper PA is fed in the sub-scanning direction (Xdirection) by a length L+P, the ink-jet head 3 is made to run in themain scanning direction (Y direction) to perform the jetting similar tothe above.

To realize 2 Ndpi (300 dpi) resolution in the sub-scanning direction byusing only the monochrome-black dots A1, then after the ink-jet head 3is made to run once in the main scanning direction (Y direction), it isnecessary to feed the paper PA in the sub-scanning direction (Xdirection) by an amount corresponding to P/2 and to further make theink-jet head 3 run once more (to perform bi-directional recording orperform uni-directional recording twice in the main scanning direction)As compared with this case, in a case in which gaps (spaces) between themonochrome-black dots A1 are filled by using the color-mixed black dotsB1, the 2 Ndpi (300 dpi) resolution in the sub-scanning direction can berealized by one running, thereby making it possible to reduce therecording time, and consequently realizing a high-speed recording.

Further, a method is also conceivable in which the monochrome-black dotsA1 are formed by using ink droplets formed in a maximum ink amountjettable from the nozzles 15 of the black ink (extra-large droplets),thereby forming a black area continuous in the sub-scanning directionwith only these monochrome-black dots A1 having the extra large size. Inthis case, the resolution in the sub-scanning direction becomes Ndpi(150 dpi), and in some case, a white streak is generated in the blackarea due to lack of an amount of the ink droplets even if theextra-large droplets are used. In this embodiment, the gaps between themonochrome-black dots A1 are filled by using the color-mixed black dotsB1, a continuous black area is formed while an amount in which the blackink is jetted to form one dot of the monochrome black is adjusted to anamount smaller than the maximum ink amount per one dot jettable fromeach of the nozzles 15 of the black ink. By adjusting the ink amount tobe smaller than the maximum ink amount for the nozzles of the black ink,it is possible to reduce electrical power and also to reduce heating ofthe piezoelectric actuator 17.

Further, by making an amount of the black ink jetted to form one dot ofthe monochrome black to be greater than the total amount of the yellowink, the magenta ink, and the cyan ink jetted to form one dot of thecolor-mixed black, it is possible to make the monochrome-black dots andthe color-mixed black dots to be uniform in size, thereby obtainingsatisfactory recording quality. At this time, an ink amount of each ofthe yellow ink, the magenta ink, and the cyan ink is adjusted to anamount smaller than the ink amount of the black ink. In this embodiment,the opening size of the nozzles 15 of the yellow ink, the magenta ink,and the cyan ink is set smaller than the opening size of the nozzles 15of the black ink. Accordingly, each of the nozzles 15 of these threecolor inks easily jets an ink droplet in a smaller amount than an amountof the black ink jetted to form an ink droplet of the black ink.

Consequently, even when the color-mixed black dots B1 are formed amongthe monochrome-black dots A1 in a mixed manner, it is possible toprevent the dots from becoming nonuniform in size due to the spreading(blurring) of the color-mixed black dot B1 on the recording medium PA.Accordingly, this improves the recording quality. Further, for thepurpose of forming the dots of the color-mixed black, the inks arejetted onto the recording medium in amount just required for forming theblack area, but not jetted in an amount unnecessarily greater than therequired amount. As a result, it is also possible to prevent excessive(unnecessary) consumption of the inks.

In order to form the color-mixed black dots, small ink droplets having asmaller size such as the intermediate droplets and the like are usedwith respect to the yellow, magenta, and cyan inks. Then it is possibleto obtain an effect of reducing the electrical driving power applied tothe piezoelectric actuator 17 to cause the jetting of these droplets,and an effect of also reducing a heat generation amount of thepiezoelectric actuator 17. Of course, the yellow ink, the magenta ink,and the cyan ink are used not only to form color-mixed black dots butalso to form color dots, and in the formation of the color dots, largeink droplets such as the large droplets are selected according to datato be recorded.

Further, in general, whereas a pigment ink is often used as the blackink, dye inks are often used as the color inks such as the yellow ink,the magenta ink, and the cyan ink. Therefore, the yellow, magenta, andcyan color inks easily blur on the recording medium to a great extent ascompared with the black ink does, and thus a dot diameter of each ofthese color inks on the recording medium becomes larger than that of theblack ink dot even if the color ink droplets and the black ink dropletare same in ink-droplet volume. Therefore, in consideration of an extentat which the color inks blur, the nozzles 15 of the black ink are formedto be greater than the nozzles 15 of the other three color inks, asdescribed above, so that each of the nozzles 15 of the color inks jetsan ink droplet in an amount smaller than that of the black-ink dropleteven if each of the black nozzles and three color nozzles are driven bythe piezoelectric actuator 17 in the same manner.

Note that the above-described color-mixed black dot is used not only ina case in which monochrome recording is instructed to the controller 30(monochrome mode) but also in a case in which color recording isinstructed (color mode) to the controller 30. In the latter case, theyellow ink, the magenta ink, and the cyan ink are jetted both to a colorarea and a black area as required.

FIGS. 6A and 6B show examples of recording states in each of which anink consumption amount is saved in the color mode. In FIG. 6A, to formthe black dots continuously in the sub-scanning direction at each timingin the main scanning direction (Y direction) of a matrix in which thedots are arranged, the monochrome-black dots A1 and the color-mixedblack dots B1 are formed adjacently in the sub-scanning direction asdescribed above. Further, dots Ca1, Cb1, Cc1, each formed by singlecolor or formed by a mixture of two colors among the other three colors,are formed at each timing at positions shifted in the sub-scanningdirection by P/2 from the positions at each of which one of themonochrome-black dots A1 is formed. In this case, a black portion, forexample, a character, in a recorded image is recorded more clearly thanin a case shown in FIG. 6B which will be described next.

FIG. 6B shows a recording state in which an ink consumption amount issaved further than that in the case shown in FIG. 6A. In this case, evenwhen the black dots are to be formed continuously in the sub-scanningdirection, the color-mixed black dots B1 are omitted.

The first embodiment is an embodiment in which the nozzle row 15 a ofthe black ink is shifted, from the nozzle rows 15 b to 15 d of the cyanink, the magenta ink, and the yellow ink, in the sub-scanning directionby ½ of the predetermined spacing distance P. It is allowable, however,that the shift amount of the nozzle row 15 a of the black ink may bearbitrary, provided that the shift amount is smaller than P. A case inwhich the shift amount is different from that of the first embodimentwill be explained as a second embodiment by using FIG. 7 and FIGS. 8Aand 8B.

In the second embodiment, as shown in FIG. 7, nozzles 15 in each of anozzle row 15 a of the black ink (black nozzle row 15 a), a nozzle row15 b of the cyan ink (cyan nozzle row 15 b), a nozzle row 15 c of themagenta ink (magenta nozzle row 15 c), a nozzle row 15 d of the yellowink (yellow nozzle row 15 d) are arranged in a straight line. In each ofthe nozzle rows, nozzles, among the nozzles 15, which are adjacent inthe sub-scanning direction (X direction) are arranged at a predeterminedspacing distance Pb (resolution in the sub-scanning direction is Mdpi,wherein M is a positive integer), and the black nozzle row 15 a isshifted, with respect to the cyan nozzle row 15 b, the magenta nozzlerow 15 c, and the yellow nozzle row 15 d in the sub-scanning directionby an amount corresponding to ¼ of the predetermined spacing distancePb. In each of the nozzle rows, a spacing distance between nozzles,among the nozzles 15, positioned in the nozzle row at both ends thereofis L.

In an ink-jet printer 1 of the second embodiment, for example, in a casein which data to be recorded includes a solid black area which iscontinuous at least in the sub-scanning direction, or in a case in whichblack dots need to be formed with 4 Mdpi resolution in the sub-scanningdirection, then an ink-jet head 3 is made to run once in the mainscanning direction (Y direction) to form monochrome-black dots A1 andcolor-mixed black dots B1 adjacently in the sub-scanning direction asshown in FIG. 8A, and thereafter, a paper PA is fed in the sub-scanningdirection (X direction) by a distance corresponding to Pb/2, and bymaking the carriage 9 run next time in the main scanning direction (Ydirection), monochrome-black dots A2 and color-mixed black dots B2 areformed in a gap (space), in the sub-scanning direction, between thepreviously formed dots, as shown in FIG. 8B.

In the second embodiment also, similarly in the first embodiment, anopening size of the nozzles 15 of the black ink is set to be greaterthan an opening size of the nozzles 15 of the other color inks, and ajetting amount of each of the color inks other than the black ink to besmaller than a jetting amount of the black ink. Accordingly, it ispossible to form the dots of the color-mixed black and themonochrome-black dots uniformly in size.

Note that, in the first embodiment, by performing control so that thepaper PA is fed in the sub-scanning direction by an amount correspondingto P/4 and that dots are further formed between the monochrome-blackdots A2 and the color-mixed black dots B2, it is possible to form ablack area with dots each having a still smaller volume (that is, withhigh resolution).

Finally, the flow of an ink jetting process in the above-describedembodiments will be briefly explained by using a flowchart in FIG. 10.When a user gives a printing instruction to an ink-jet printer providedwith the ink-jet head described in the above-described embodiments (S1),a judgment is made whether or not the instruction from the user is aninstruction for high-resolution recording (S2). If the high-resolutionrecording is instructed (S2: Yes), a color-mixed black dot is formedbetween monochrome-black dots adjacent in the sub-scanning direction, byconcurrently jetting the yellow ink, the magenta ink, and the cyan ink(S3). At this time, a jetting amount of the black ink to form onemonochrome-black dot is set to be greater than the total volume of theyellow ink, the magenta ink, and the cyan ink jetted to form onecolor-mixed black dot (S4). In a case in which the high-resolutionrecording is not instructed (S2: No), the recording is performed byjetting only the black ink (S5).

The ink-jet printer and the ink jetting method of the present inventionhave been specifically explained. The present invention is, however, notlimited to these embodiments and any modification and change may be madewithin the scope of the claims. For example, the ink-jet head is notlimited to a piezoelectric type, and resistance wires, which generateJoule heat in response to a driving signal, may be provided in inkchannels respectively. In this case, the inks in the ink channels areheated to be vaporized, thereby generating air bubbles, and the inks canbe jetted by the pressure of the air bubbles.

1. An ink-jet printer which performs recording on a recording medium by jetting a black ink, a yellow ink, a magenta ink, and a cyan ink, the ink-jet printer comprising: an ink-jet head including black, cyan, magenta and yellow nozzle rows each of which is formed of a plurality of nozzles arranged in a predetermined direction, and which are aligned in a direction orthogonal to the predetermined direction; and a controller which controls the ink-jet head; wherein adjacent nozzles, among the nozzles in each of the nozzle rows, are arranged at a predetermined spacing distance P; the black nozzle row is shifted, in the predetermined direction by a spacing distance ΔP smaller than the predetermined spacing distance P, from the cyan, magenta and yellow nozzle rows, respectively; when a black dot is formed between monochrome-black dots of monochrome black which are adjacent to each other in the predetermined direction and which are formed by jetting the black ink, the controller controls the ink-jet head so that the yellow ink, the magenta ink, and the cyan ink are jetted from nozzles to form a color-mixed black dot of color-mixed black which is a mixture of the yellow ink, the magenta ink, and the cyan ink; and the controller controls the ink-jet head so that an amount of the black ink which is jetted to form one dot of the monochrome-black dots is greater than a total amount of the yellow ink, the magenta ink, and the cyan ink jetted to form the color-mixed black dot.
 2. The ink-jet printer according to claim 1, wherein the black nozzle row is shifted, in the predetermined direction by ½ of the predetermined spacing distance P, from the cyan, magenta and yellow nozzle rows, respectively.
 3. The ink-jet printer according to claim 1, wherein the nozzles which form the black nozzle row and which jet the black ink have an opening which is greater in size than that of the nozzles which form the cyan, magenta and yellow nozzle rows and which jet the cyan, magenta and yellow inks, respectively.
 4. The ink-jet printer according to claim 3, wherein, when the color-mixed black dot is formed between the monochrome-black dots, an amount of the yellow ink, an amount of the magenta ink, and an amount of the cyan ink jetted to form the color-mixed black dot are each adjusted to be smaller than the amount of the black ink jetted to form one dot of the monochrome-black dots.
 5. The ink-jet printer according to claim 3, wherein an ink, which blurs more easily with respect to the recording medium than the black ink, is selected in advance as each of the yellow ink, the magenta ink, and the cyan ink.
 6. The ink-jet printer according to claim 1, wherein the amount of the black ink jetted to form one dot of the monochrome-black dots is 1.3 to 1.5 times an amount of each of the yellow, magenta, and cyan inks jetted to form one dot of each of the yellow, magenta, and cyan inks in a size same as that of one dot of the monochrome-black dots.
 7. The ink-jet printer according to claim 1, wherein when the color-mixed black dot is formed between the monochrome-black dots, the amount of the black ink jetted to form one dot of the monochrome-black dots is smaller than a maximum ink amount jettable for forming one dot of the monochrome-black dots from each of the nozzles which form the black nozzle row and which jet the black ink.
 8. An ink jetting method for jetting a black ink, a yellow ink, a magenta ink, and a cyan ink from, the method comprising: preparing an ink-jet head which includes black, cyan, magenta and yellow nozzle rows each formed of a plurality of nozzles arranged in a predetermined direction at a predetermined spacing distance P, and are aligned in a direction orthogonal to the predetermined direction, and in which the black nozzle row is shifted, in the predetermined direction by a spacing distance ΔP smaller than the predetermined spacing distance P, from the cyan, magenta and yellow nozzle rows, respectively; jetting the yellow ink, the magenta ink, the cyan ink concurrently so as to form a color-mixed black dot of color-mixed black which is a mixture of the yellow ink, the magenta ink, and the cyan ink, when a black dot is formed between monochrome-black dots of monochrome black which are adjacent to each other in the predetermined direction and which are formed by jetting the black ink; and adjusting an amount of the black ink to be greater than a total amount of the yellow ink, the magenta ink, and the cyan ink jetted for forming the color-mixed black dot, when one dot of the monochrome-black dots is formed.
 9. The ink jetting method according to claim 8, wherein the black nozzle row is shifted, in the predetermined direction by ½ of the predetermined spacing distance P, from the cyan, magenta and yellow nozzle rows respectively
 10. The ink jetting method according to claim 8, wherein the nozzles which form the black nozzle row and which jet the black ink have an opening which is greater in size than the nozzles which form the cyan, magenta and yellow nozzle rows and which jet the cyan, magenta and yellow inks, respectively.
 11. The ink jetting method according to claim 8, wherein an ink, which blurs more easily with respect to the recording medium than the black ink, is selected in advance as each of the yellow ink, the magenta ink, and the cyan ink. 